Synopsis: The limits of a closed shell

The noble gas atoms, like xenon and argon, are inert because their electrons form a closed shell. Similarly, the filling of proton or neutron shell states in atomic nuclei has a stabilizing effect. New data are, however, showing that even a closed shell structure is a fragile boundary, especially for weakly bound, exotic nuclei far from the valley of stability.

Lead is an attractive element to study these effects. With 82 protons, lead has a closed shell structure for the protons. Lead also has many accessible isotopes, which allows experimentalists to measure how the binding effects of a closed shell structure weaken in nuclei with progressively fewer neutrons.

One reason that the closed shell weakens in neutron-deficient elements is that attractive interactions between valence protons and neutrons in spatially overlapping orbits lower the energy of certain proton excitations. A light lead nucleus with these proton excitations has a nonspherical, or deformed, shape that is different than the normal states of the nucleus. To see this “shape coexistence,” however, requires highly sensitive spectroscopy of the nuclear states.

Now, in a Rapid Communication appearing in Physical Review C, a collaboration between Finland, the UK, France, and Belgium reports a gamma-ray spectrum of 180Pb—the most neutron deficient isotope of Pb yet studied with spectroscopy. They first detected the gamma rays from a variety of nuclear reactions and then identified those gamma rays coming from 180Pb nuclei. To do this, they measured the characteristic alpha decay of 180Pb at the focal plane of a magnetic spectrometer. The experiment was a true tour-de-force as the cross section for producing 180Pb is exceptionally small (of order 10 nanobarns).